The temperature of a tube is often indicative of the temperature of a fluid flowing through the tube, especially in case the tube is made of a temperature-conductive material. Temperature measurement of a tube is therefore often used for determining the temperature of the fluid inside the tube as this is normally easier than measuring the temperature of the fluid itself. Accordingly, a reliable temperature measurement system is needed for measuring the temperature of a tube.
Flowmeters are used to determine the mass flow rate of a fluid flowing through a tube. Coriolis mass flowmeters use the vibration pattern of two measuring tubes arranged in parallel for this purpose. The stiffness of these measuring tubes is dependent on the coefficient of elasticity of the tube material and the thermal expansion of the measuring tubes. Accordingly, the vibration pattern also, to some extent, changes with temperature. Coriolis mass flowmeters therefore take temperature measurements of the measuring tubes in order to compensate the temperature dependency and to obtain correct measurement results. Without temperature correction, measurement errors of about 5% would occur for a temperature change of 100° C. The deviation is the higher the more the measurement temperature differs from the temperature at which the flowmeter was calibrated. As calibration is normally carried out at room temperature this means that the deviation increases with increasing temperatures of the measuring tubes. At high and low (cryogenic) temperatures, reliable compensation—and thus correct mass flow rate measurement-primarily depends on a correct temperature measurement of the measuring tubes.
The measuring tubes utilized in flowmeters are usually rather small so that penetrating them with a temperature probe to measure the internal temperature is generally impractical. Some devices are known where the temperature probe is inserted into the flow splitter of the flowmeter. While this arrangement is very complicated and expensive the temperature measurement is still prone to errors. On the other hand, measuring the temperature at the outer surface of the measuring tube is easier to handle but often leads to unreliable and/or incorrect results. This is what can widely be experienced with prior art Coriolis mass flowmeters using a temperature probe which is directly attached to the tube surface. The temperature probe used is usually a resistance temperature detector (RTD) which is glued, soldered or brazed to the outer surface of the measuring tube. The heat dissipation of the RTD outer surface, which is normally not in a very good thermal contact with the tube, greatly influences the measured temperature. Measurement errors in the range of from twice to 5-fold as high as the specified accuracy can be seen at, for example, 125° C. as compared to 25° C. ambient temperature.
It is therefore an object of the present invention to eliminate the above-mentioned problems and, more precisely, to devise a temperature measurement system for measuring a temperature of a tube which can easily be installed at the outer surface of the tube and without the need to insert a temperature probe into the tube interior while leading to reliable and reproducible temperature measurement results. A further object is to devise a flowmeter, particularly a Coriolis mass flowmeter, utilizing a temperature measurement system for measuring a temperature of a measuring tube which allows for an improved temperature compensation of the mass flow rate measurements.